1. Field of the Invention
The present invention relates to an image developing device for use in an electrophotographic apparatus or the like.
2. Description of the Prior Art
Some conventional image developing devices for use in electrophotography include a developing roller for transferring a thin layer of toner to a latent image carrier and a toner supply roller held against the developing roller and rotatable independently thereof. The toner supply roller is usually made of a high-resistance resilient material such as sponge (having a volume resistance of 10.sup.10 ohms/cm or more). However, the toner supply roller made of such a material fails to scrape residual toner efficiently off the circumferential surface of the developing toner. If the toner supply roller were unduly pressed against the developing roller or rotated at an increased speed for higher toner scraping capability, the torque required to drive the toner supply roller would be increased or a residual image would be produced.
To avoid the aforesaid problem, it has been customary to apply predetermined bias voltages to the respective rollers used for developing images. Since, however, different bias voltages are required to be impressed on the corresponding rollers, separate circuits have heretofore been needed in the image developing device for applying the respective bias voltages to the rollers. The separate circuits in turn require a plurality of power supply circuits. This prior circuit arrangement is costly and requires a large installation space, with the result that the image developing device becomes inevitably large in size. Inasmuch as the bias voltages to be applied to the rollers are fixed for the toner being used, they cannot be changed to other voltages selected to best match a toner of a different color that is desired to be used in place of the existing toner.